It really comes down to getting more battery life for the dollar with the 6v golf cart batteries and the reason for 6v is to keep the weight down so a person can lift them into place. But neither golf carts or forklifts where these are commonly used have only 2 batteries installed.
With an RV if one cell is bad in one of the 6v batteries you no longer have 12v which is needed to operate any electronic devices including the fridge and the furnace thermostat. If you have 4 batteries then you can afford to lose one and still have 12 volts possible until you can replace the battery with the bad cell.
As a practical matter I use two 12v batteries wired in parallel so if one fails I lose some amps but still have 12 volts. The most we have ever discharged them is 22% so even with one battery that would still take keep it above 50% DOD.
